Such devices are also referred to as “expanders” or expander heads, and they serve, among other things, as tools for use in plumbing. The device can be driven by hand, for example, by a manual lever, or else by a motor using threaded spindles, or by hydraulic machines.
European patent specification EP 0 725 908 B1 and the corresponding translation of the European patent DE 694 14 721 T2 describe that, when plastic pipes are expanded in order to create pipe connections, conditions other than in the case of expanding metal pipes have to be additionally observed. Plastic pipes, especially those made of crosslinked polyolefins, including crosslinked polyethylene, have a so-called shape memory, which is also referred to as the memory effect. This means that, after the pipes have been expanded, they contract again over the course of time, so that appropriate working speeds have to be employed. Moreover, the walls of plastic pipes are much thicker than those of metal pipes of the same diameter, so that the radial expansion has to correspond at least to these wall thicknesses.
On the basis of these considerations and before the backdrop of preventing sharp-edged impressions caused by the edges of the expandable jaws—which would lead to leaks in the later pipe connections—the above-mentioned documents propose carrying out the expansion in several stages, returning the expandable jaws to the starting position after each stage, and then pushing them deeper into the end of the pipe and repeating or continuing the expansion several times. In this process, the stages should be radially much smaller than the wall thickness of the pipes. The radial movement is made by an expansion mandrel having a slender conical surface that is pushed into a set of, for instance, six expandable jaws. These expandable jaws each have two outer cylindrical surfaces between each of which there is a conical surface in the axial direction, and these match the surface geometries of the expanded jaw position. The outsides of the conical surfaces can also be provided with fine teeth.
On the one hand, this makes it possible to select the circumferential surfaces of the jaw sectors to be larger although at the expense of the gap widths between the jaws, but on the other hand, this design requires multiple tool actuations for each pipe connection and this—also because of having to shake the tool loose—is time-consuming and tiring for the operator in view of the force needed, as well as because of the weight of the tool. Furthermore, the expanded pipe end also retains its funnel shape, even when it is being pushed over the last cylindrical surface, as a result of which the guiding properties of the pipe end are partially lost.
East German patent application DD 221 936 A1 and German patent DE 43 39 993 C1 describe, for tools with a one-stage expansion, to arrange alternating projections and recesses in the joints between the expandable jaws in the axial direction in order to lessen the collapsing of the wall of the hollow body as the radial movement of the expandable jaws increases. Here, it is kept in mind that the outer surfaces of the expandable jaws, the so-called working surfaces, should be in the radial final position in a cylindrical surface. However, it has been found that this effect is accompanied by undesired deformations. Therefore, German patent specification DE 43 39 993 C1 proposes beveling the ends of the projections vis-à-vis the partially cylindrical working surfaces towards the inside in such a way that, in the contracted state of the expandable jaws, the outer surfaces of the projections do not project very much beyond the cylindrical partial surfaces that are defined by the working surfaces.
However, it has been determined in the present invention that, at the beginning of the expansion, the edges formed between the cylindrical surfaces and the bevels leave behind sharp impressions that cannot be made to disappear completely during the course of the further expansion. The cause of this can be assumed to be that the edges cause bending and thus a greater flow of the material, as a result of which the strength of the material is increased excessively locally and this can no longer be compensated for when the expansion is continued. This effect is referred to in metallurgy as cold hardening (see German encyclopedia “Grosse Brockhaus”, 1983, p. 138). As a result, the connection sites of the pipe ends are not tight, and this leads to leaks.
European patent application EP 0 718 057 A2 describes breaking or rounding off the axis-parallel lengthwise edges as well as the circular sector-shaped front edges of the expandable jaws, specifically with a maximum radius of 2.0 mm. However, in actual practice, it has been determined in the present invention that such rounding off of the front edges is not sufficient to prevent brittle fractures of the plastic pipes under mechanical stress, be it due to the expansion while pipe sections are being joined, due to deformations of the pipe during installation, or due to high internal pressures such as those encountered in water pipes used for drinking water or heating.